Heating systems and air conditioning systems are generally operable in response to a thermostat. The thermostat opens or closes in response to the surrounding temperature and acts to maintain the heating and air conditioning system in operation until a pre-set temperature is reached. Both heating and air-conditioning systems operate efficiently only for a short period.
In a furnace the temperature of the heat exchanger increases as fuel burns. Eventually the heat exchanger surface in contact with the fire box becomes so hot that the heated air is forced from the fire box through the chimney without raising the temperature of the heat exchanger and thus energy is wasted.
For this reason, it is preferable that the heating source of a furnace periodically shut down while air is forced through the heat exchanger causing the surface to cool down. This permits heat from the fire box to efficiently transfer heat to the heat exchanger.
With an air-conditioning system, the compressor operates and causes coils within the plenum to cool to a minimum of about 34.degree. C. Even though the coils are at their minimum temperature, the compressor continues to operate using energy but without value.
A solution to this problem has been to incorporate a duty cycling switch with the furnace or air conditioner. Various types of these are known, for example, Kinsey U.S. Pat. No. 3,136,730 discloses a method to improve furnace efficiency by running the burner for only a pre-set period of time and then restarting the burner after a second pre-set period of time. This may be effective for new units which can be designed for a particular house and a particular plenum. But establishing what the pre-set periods of time for an existing unit is very difficult. Changes in demand for heating or cooling make these preset periods of time very inaccurate.
Hamilton U.S. Pat. No. 3,921,899 discloses a temperature sensing duty cycling switch which attaches to the plenum and controls the flow of gas to the burner of a furnace in response to the temperature inside the plenum. When the temperature is below a certain temperature, the burner can be activated. If the plenum temperature exceeds a certain temperature the burner is deactivated. Although the system disclosed in the Hamilton reference measures temperature, because the switch utilizes a temperature probe inserted into the plenum, it is excessively responsive to rapid temperature fluctuations within the plenum. Further, there are substantial differences in the temperature of the air within the plenum at various locations. The system disclosed in Hamilton is totally ineffective for retro-fitting existing units. Brown U.S. Pat. No. 4,534,181 discloses the use of a duty cycling switch with an air conditioner.
A duty cycling switch disclosed in Davis et al U.S. Pat. No. 4,470,267 is specially designed to retro-fit existing units. It is designed to mount to the exterior wall of a furnace or air conditioning plenum and opens and closes in response to the temperature of the plenum wall. The switch can be adjusted to be deactivated at a desired temperature preferably the temperature at which the furnace or air conditioner is operating most efficiently. More particularly this switch is a bi-metal duty cycling switch which is enclosed within a case wherein the base of the case is metal. The bi-metal switch is in direct contact with the metal base plate which in turn contacts the plenum wall. This switch has been found to be effective to efficiently duty cycle air conditioners and furnaces.
The unit disclosed in the Davis Patent is unfortunately ineffective where excessive temperatures are experienced on the exterior of the plenum wall. This can occur in various situations and is particularly encountered with roof mounted heating or air conditioning units. In many homes and buildings, the heating or air conditioning units are mounted directly to the roof of the building. The ducts go directly through the roof into the home. There is no accessable portion inside the attic to mount a duty cycling switch. Mounting it to the exterior of the duct exposed on the roof makes the switch responsive to exterior ambient temperature as opposed to the temperature of the furance or air conditioner plenum.
If a bi-metal switch is simply located inside the duct it suffers from the sam disadvantages of the duty cycling switch disclosed in the Hamilton reference. Due to the rapid fluctuation of air temperature as well as the variation of air temperature within the duct the switch would be given to false readings and therefore it would be ineffective.
As previously stated due to excessive external temperatures, it cannot be mounted to the exterior of the duct exposed to ambient temperatures.